The present invention relates to a convertor valve for high voltages. Air-insulated types of these convertor valves are normally constructed in high valve stacks. These are composed of a number of electrically connected valve modules which may be arranged in floors one above the other with one or more valve modules on each floor. Each valve module comprises convertor valves which may be of thyristor type as well as auxiliary equipment such as voltage divider circuits in which, inter alia, oil-impregnated capacitors may be included. The valve modules may also be arranged in other ways in the valve stack; however, at some location in the stack a valve module will always be positioned above another valve module. A large fire in a valve module may therefore spread upwards to a valve module positioned above it in a valve stack.